This application claims priority from German application no. 200 21 398.9 filed Dec. 18, 2000.
The present invention relates to an exhaust gas purification system for denoxing exhaust gases (i.e. removing oxides of nitrogen) of a combustion unit on an SCR catalyst utilizing ammonia (NH3), additionally introduced into the exhaust gas flow before the SCR catalyst.
Apart from carbon monoxide (CO) and hydrocarbons (HC), in particular nitrogen oxides (NOx) are among the environmentally harmful, directly emitted primary injurious substances which are generated during operation of internal combustion engines, in particular Diesel engines. Such nitrogen oxides include such compounds as nitric oxide, nitrogen trioxide (N2O3), dinitrogen tetroxide (N2O4), nitrogen dioxide (NO2) and trinitrogen tetroxide (N3O4). The use of three-way catalysts, such as are used in Otto engines and gas engines, cannot be used in the exhaust of Diesel engines due to an excess of oxygen. For this reason, for the reduction of the nitrogen oxides emission in Diesel engines a selectively working SCR catalyst (Selective Catalytic Reduction catalyst) was developed, in which the expelled nitrogen oxides are reduced to N2 and H2O in the presence of a supplied reducing agent, namely ammonia (NH3).
Such exhaust gas purification systems are known, for example, from EP 0 277 765 A1 or EP 0 381 276 A1. The NH3 is stored as a fluid in a pressure-tight container and connected with the exhaust gas line via a supply line. From the reducing agent container the gaseous NH3 is tapped, then injected in the required quantity into the exhaust gas flow via a metering valve driven by a control unit. Metering of the NH3 quantity takes place as a function of the instantaneous operating states of the combustion unit. These patents are incorporated herein by reference.
In these prior known commercially available systems, pressure-tight gas cylinders are employed as reducing agent containers. The containers are connected in conventional manner to the supply line by means of a screw connection. When using such a prior known exhaust gas purification system in a motor vehicle, for example in a commercial motor vehicle, the intent is to provide the employed reducing agent container for reasons of safety, but in particular also for reasons of handling techniques, such that it is not too large. Therefore the reducing agent containers in such exhaust gas purification systems applied in a motor vehicle must be replaced frequently. For reasons of handling techniques, however, for replacing such a container it is not appropriate, and correspondingly time-consuming, if with one or several NH3 gas cylinders carried on board, first, all connections must be unscrewed and subsequently newly emplaced filled reduction containers must be screwed on. Moreover, in the case of such a conventional connection of prior known containers, it is a disadvantage that the person replacing the reducing agent container by necessity is in the direct proximity of the free end of the supply line so that it is possible that he is in contact with the gaseous NH3 flowing out of the supply line.
Building on this discussed prior art, the present invention is therefore based on the task of developing further an exhaust gas purification system according to the species described in the introduction, such that the disadvantages demonstrated regarding prior art are minimized if not even avoided.
This task is solved according to the invention, for one, wherein that at least one reducing agent container is held in a docking device as part of the exhaust gas purification system and, via a coupling device, is connected with the supply line, of which coupling device one coupling part is associated with the docking device and the other coupling part with the container, wherein both coupling parts can be engaged with an axial movement directed toward one another, and the docking device and the container comprise cooperating guide means through which a container under guidance can be brought into its specified position for joining the two coupling parts.
The task is further solved through a system or configuration for holding the reducing agent container(s) in the frame of a replacement container system and connecting it to an exhaust gas purification system such as cited in the introduction, which comprises a container changing system comprising a docking device and a two-part coupling device, the coupling device comprising one coupling part associated with the docking device and the other coupling part associated with at least one reducing agent container which is held in the docking device, wherein the two coupling parts can be engaged with one another. The docking device and the reducing agent container further comprise cooperating guide means, through which the reduction container(s) can be brought into the specified position (under guidance) for engaging the two coupling parts. Furthermore, this task is solved with a reducing agent container for application in an exhaust gas purification system such as cited in the introduction, comprising a cylindrical shell with one coupling part disposed on the head end of the container, while at the foot end handle means are disposed for handling the container for insertion into a docking device and during engagement of the two coupling parts.
The exhaust gas purification system according to the invention comprises a docking device for receiving at least one reducing container. This docking device includes a coupling part, which can be engaged with a second coupling part, associated with the reducing agent container, for the purpose of establishing a fluid connection between the container and the supply line. The two coupling parts are laid out or arranged in the manner of a quick connect coupling and can be engaged with one another via an axial movement, at least as a component, directed toward one another. To connect the two coupling parts, it can therefore be sufficient to simply move the two coupling parts axially toward one another until the specified connection is established. It can also be provided that the axial movement represents only a vectorial component of a movement toward one another of the two coupling parts, in which also a rotational component is included. In such a case the two coupling parts can be engaged for example by a bayonet coupling. The docking device further comprises guide means through which a container can be brought into its specified position for joining the two coupling parts. The guide means can comprise such means which serve for guiding and supporting the container as such and can comprise further guide means which are provided to place the two coupling parts for engagement with one another as specified.
In the case of an exhaust gas purification system equipped with such a docking device, not only can a quick change of the applied reduction container take place, but it is in particular possible to connect through the applied guide means the container(s) to the supply line without anyone needing to directly guide the coupling parts themselves for joining the coupling parts. Consequently, such containers can be connected to the exhaust gas purification system via the docking device whereby the reduction container is handled at its end opposing the coupling part and the person carrying out the container replacement thus is located at a relatively great distance from the two coupling parts which have to be placed for engagement with one another. For this purpose a reducing agent container for such an exhaust gas purification system comprises at its foot end, opposing the coupling part, handle means for handling the reduction container. As a handle can be provided, for example, a ring with grip openings included therein and encircling the base of the container. For this purpose the containers are conveniently held in the horizontal position in the docking device.
To absorb the vibrations of the reduction container during the operation of a motor vehicle, with respect to the supply line and the units connected into the supply line, the coupling part associated with the docking device is usefully fastened on the docking device so as to be decoupled with respect to stresses. This can, for example, be realized through a floating bearing of the coupling part and through a tube connection between the coupling part and the supply line.
If the exhaust gas purification system comprises guide means for placing the two coupling parts into engagement with one another, which are developed in the manner of a bayonet coupling or lock, it is useful to provide these in annular bodies disposed concentrically with one another, wherein, associated with the container, is one such annular body encircling concentrically the coupling part associated with it with one or more top-side open bayonet grooves introduced therein and a further annular body disposed concentrically with the coupling part associated with the docking device with a number of guide pins corresponding to the number of bayonet grooves. Utilization of such a bayonet lock between the reduction container and the docking device enables a guided movement with which an axial displacement as well as also a rotational movement of the two coupling parts with respect to one another can be carried out. It can therein be provided that the applied guide grooves in a terminal section do not have a slope which is useful when, as the coupling is applied, and after the specified connection is established through a final rotational movement of the two coupling parts relative to one another, an actual opening takes place to establish the desired fluid connection.
A further embodiment of such exhaust gas purification system provides that the reducing agent container(s) set into the docking device additionally are locked through a locking device to secure the containers placed in the docking device. When using such a locking device, it is useful to provide means which permit closing the locking device only if the container(s) set into the docking device is(are) disposed therein as specified. Such additional securing means can be realized, for example, when the locking device comprises a locking arm pivotably articulated on the docking device, wherein such a locking arm is assigned to each reducing agent container set into the docking device. Further disposed on the locking arm are extensions which, at a specified positioning of the container assigned to the locking arm, extend into corresponding openings or recesses of the container. When using a reducing agent container as described above with a handle disposed on the base, such openings can be included, for example, in the lower rim of the handle.
The exhaust gas purification system can be configured to remove NH3 from a reducing agent container either in its gaseous or in its liquid phase. It can be provided that NH3 is contained in the reducing agent container in its pure form, in a mixed form or also chemically bound, from which chemical compounds it can be split off thermolytically. If the exhaust gas purification system is laid out or adapted for removing NH3 gas from the reduction container, it is useful to associate a heat source with the docking device, which is implemented and arranged such that only a subregion of each reducing agent container or its contents will be heated by the heat source.
By heating the liquid NH3 contained in the container, the vapor pressure in the reduction container increases correspondingly so that even at low ambient temperatures a satisfactory gas pressure is provided in order to provide a sufficient quantity of gaseous NH3 for the exhaust gas purification. However, by heating the contents of the container it is also possible to empty the container completely. The same applies when a relatively large quantity of NH3 is removed at one time from the container. In the claimed exhaust gas purification system the heat source is configured and disposed such that through the heat source only a subregion of the reduction container or its contents will be heated, and specifically such subregion of the reducing agent container in which the liquid NH3 is contained, until it is emptied. As a rule, this is the furthest reaches of the container. Heating a subregion of the container is especially simple of realization since for this purpose only a low heat quantity is required and in an especially sophisticated manner the convection, occurring in the reduction container with local heating of the liquid NH3, and with local boiling the turbulence induced through vapor bubble formation are utilized for heat distribution within the liquid contained in the container. Therefore in this exhaust gas purification system it is not necessary to heat the entire reduction container. Moreover, when applying a heat source causing only local heating, a control circuit responding significantly more rapidly can be developed, and employed.
The heat source employed can be, for example, a resistance heater or also as PTC element on the surface of the reducing agent container.
However, in a preferred embodiment of such an exhaust gas purification system the heat source is laid out or adapted for heating the contents of the reducing agent container from the outside, and free of contact. The advantage of such heating, which is contact-free with respect to the container, of its contents lies in that in this case the reduction container can be moved, in a simple manner after it has been emptied, from the frame of the described exchange system using the docking device. The empty container can then be replaced by a filled one without an additional subsequent mounting or setting up of the heat source having to be carried out. As a contact-free operating heat source can be provided, for example, one such which emits high-energy radiation wherein only a subregion of the reduction containerxe2x80x94the local heating sitexe2x80x94is located within the radiation of the heat source. The heat source can be configured out, for example, to emit IR (infrared) radiation, and with such an implementation it is useful to adapt the local heating site of the reduction container such that it is especially radiation absorptive, for example by being blackened. According to a further embodiment, the heat source for heating the liquid NH3 can also be a transmitter for emitting microwave radiation as the high-energy radiation. It is understood that a subregion of the reduction container can also be irradiated or heat-activated by several other discrete heat sources. To facilitate the handling of the reducing agent container when it is heated, the handle is produced of a material with low thermal conductivity, for example of synthetic material.
When using such a heat source for heating the liquid NH3 in the reducing agent container it is useful to equip the inner surface of the container in the region of the local heating site with means, for example roughening of the inner surface, which promote the formation of vapor bubbles,
Formation of vapor bubbles can also be promoted with boiling chips in the case of such a surface such that an intensive heat distribution occurs through the turbulence generated with vapor bubble formation.
With such a configuration it is further useful if the output of the reducing agent container is succeeded by a liquid trap such that any liquid escaping during a change of a reducing agent container is collected in the liquid trap and does not penetrate into the further system. Such a liquid trap can also comprise a heating device to allow the NH3 collected in the liquid trap to be vaporized and to supply the NH3 in its gaseous phase to the provided exhaust gas purification system.
In case the exhaust gas purification system is laid out or adapted to remove NH3 in its liquid phase from the reducing agent container, it is useful that a metering device is connected to the removal device in the direction of flow of NH3 in the supply line and, succeeding the metering device, a vaporizer before the supply line ends in the exhaust gas line.
Due to this measure the removal of NH3 is not dependent on a certain gas pressure in the reducing agent container such that NH3 can be removed from the container at any time, and in particular under cold ambient conditions. When removing liquid NH3, a nearly complete emptying of the reducing agent container is also possible. For this purpose the reducing agent container comprises a removal device for removing liquid NH3, which can be developed, for example, in the manner of a nozzle tube. The entrance of liquid ammonia into such a removal nozzle tube can be realized through the vapor pressure in the container or also through a suction device.
The removal device for removing liquid NH3 from the reducing agent container is succeeded in the direction of flow by a metering device, for example a metering valve, with which the particular required quantity of NH3 is removed from the container. Succeeding the metering device is a vaporizer for vaporizing the NH3 removed by the metering device from the reducing agent container. It can be provided that the meter device supplies NH3 timed or continuously, in the particular required quantity, to the vaporizer. The vaporizer vaporizes the liquid NH3 for injection into the exhaust gas line in the gaseous state.
In the case of an exhaust gas purification system operating according to the principle of the invention, vaporization of the liquid NH3 takes place only in such a quantity as is in fact currently required in the exhaust gas line for carrying out the desired denoxing of the invention. It is therefore not necessary to configure the applied vaporizer such that it simultaneously serves as a storage container for a relatively large quantity of gaseous NH3, from which the required quantity is subsequently removed for injection into the exhaust gas line. Consequently, the necessary safety requirements for the vaporizer are minimized.
To improve the safety of the exhaust gas purification system in the event of a failure of the vaporizer and in the event of a defective, open and no longer closable metering device, a further development provides a device for hindering or preventing the escape of liquid NH3 disposed at the output of the vaporizer. Such a liquid barrier can be realized, for example, with a float valve which closes as soon as liquid is present at the output of the vaporizer. To hinder the escape of liquid from the vaporizer, however, a filter element can also be provided which allows gaseous NH3 to penetrate but very little or no liquid NH3. When using a float valve it is useful to provide it with appropriate sensors or switches such that the position of the float can be detected by a control device. If the system detects that the float valve is closed, which is equivalent to the presence of liquid NH3 at the output of the vaporizer, for example, an emergency valve can be closed or the vaporizer can be started in order to convert the accumulated liquid NH3 into its gaseous and thus less hazardous phase. Such a barrier device equipped with sensors can also switch the vaporizer directly on or off.
Succeeding the vaporizer in a preferred further embodiment is an air supply device through which additional air is supplied to the supply line connecting the reducing agent container with the exhaust gas line. This supplied air can serve in continuous operation of the exhaust gas purification system for increasing the transport rate of the gas generated in the vaporizer into the exhaust gas line and/or for flushing the supply line. Such flushing is useful in particular when a relatively long transport path to the exhaust gas line must be traversed between the vaporizer or, when using a liquid barrier device, between the liquid barrier device and the exhaust gas line.
It is useful to accommodate all of the units necessary for removing NH3 in the docking device since, as a rule, it is protected against external influences. In this case only the terminal section of the supply line is between the docking device and the exhaust gas line. Such a configuration is useful in particular also for retrofitting present combustion units, since the critical components are all integrated in one modulexe2x80x94the docking devicexe2x80x94and lastly only one line must be carried to the exhaust gas line.
Other aspects of this invention will appear from the following description and appended claims, reference being made to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views.